1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a zoom lens system for use in an image projecting apparatus such as a microfilm reader or a microfilm reader/printer which projects microfilm images and has a limited conjugate length.
2. Description of the Prior Art
A microfilm includes a plurality of images thereon which are not always recorded in the same direction. Therefore, an image projecting apparatus such as a microfilm reader or a microfilm reader/printer has a prism disposed between a projecting lens and an image receiving plane for optically rotating images. This prism normally is disposed adjacent an enlargement side of the projecting lens for compactness.
In such an image projecting apparatus, as shown in FIG. 2, an image of a microfilm held by a microfilm holder 1 comprising two flat glass plates is projected onto a screen 7 through a projecting lens system 2, an image rotating prism 3 and mirrors 4-6. A dove prism may be used as the image rotating prism 3 for compactness of the prism itself. In order for the entire projected image to enter this prism 3, it is desirable to arrange the projecting lens system 2 and prism 3 as close to each other as possible, and to arrange the pupil position of the projecting lens 2 as close to the prism 3 as possible.
Zoom lens systems used as such a projecting lens system of the image projecting apparatus are disclosed in Japanese Patent Publication Kokai No. 62-237414, U.S. Pat. No. 4,733,951, U.S. Pat. No. 4,745,204, U.S. Pat. No. 4,743,102 and U.S. Pat. No. 4,750,820.
The zoom lens system disclosed in Japanese Patent Publication Kokai No. 62-237414, as shown in FIG. 3B, is a positive-negative, two group zoom lens system comprising a positive lens group 8 at an enlargement side and a negative lens group 9 at a reduction side. An aperture stop S is fixedly disposed at the enlargement side of the lens system. Reference F denotes a microfilm plane. With such a positive-negative, two group zoom lens system, however, when the zoom ratio is increased to twofold, the positive lens group moves a great distance with a zooming operation and, since the aperture stop S is fixed, a main beam of a maximum angle of view has a very large incident angle .alpha.O with respect to the positive lens group 8 at a shortest focal length side. This entails an increase in the outer diameter of the lenses and difficulties in compensating for aberration. U.S. Pat. No. 4,733,951 also discloses a positive-negative, two zoom lens system which has a similar drawback.
On the other hand, the zoom lens systems disclosed in U.S. Pat. No. 4,746,204, U.S. Pat. No. 4,743,102 and U.S. Pat. No. 4,750,820 are negative-positive, two group lens systems comprising a negative lens group at an enlargement side and a positive lens group at a reduction side. Such a negative-positive lens system has a wide angle of view but, because of its construction, has a principal point located relatively close to the reduction side, and hence the disadvantage of having an increased overall length of the lens system.